1. Field of the Invention
This invention generally concerns surface emitting lasers, and is specifically concerned with a long wavelength, indium phosphide (InP) based vertical cavity surface emitting laser having a high-reflectivity distributed Bragg reflector.
2. Related Background Art
Vertical cavity surface emitting lasers (VCSELs) are well known in the prior art. Generally speaking, a VCSEL is a semiconductor device comprising a lower substrate, a distributed Bragg reflector (DBR), an active layer (or layers) where the lasing occurs, and an upper reflector which may be either another DBR or other type of mirror. Upper and lower electrode layers are also provided in the device. An opening is centrally provided in either the upper or lower electrode layer to admit light out of the device generated by the active layer when an electric voltage is applied across the upper and lower electrode layers.
VCSELs possess many attributes that make them well suited for use in low-cost optical communication networks. The symmetrical, circular beam emitted from the VCSEL structure may be easily coupled to optical fibers. The VCSELs' small size allows them to be densely arrayed along two dimensions on a single semiconductor wafer. Finally, unlike other types of lasers, such as edge emitting lasers, there is no need to mechanically or chemically create a separate, reflecting facet to act as a resonating mirror is integrally grown within the laser in the form of a DBR, which advantageously reduces the manufacturing cost.
While VCSELs for optical networks may be formed from a number of different semiconducting materials, indium phosphide (InP) is preferred, as the range of laser wavelengths generated by such VCSELs (which is between 1.1 and 2.0 microns) encompasses both of the relatively long wavelengths of light presently best transmitted through optical fibers in such networks, i.e., around 1.31 microns and 1.55 microns.
In the past, it has proven particularly difficult to manufacture InP-based VCSELs that are capable of efficiently generating laser radiation having a center wavelength of around 1.3 microns. This problem is caused by the failure of the DBR within the VCSEL to provide the high degree of reflectivity (99.9% desired) necessary to produce an efficient lasing operation. To understand the origin of this problem, some discussion of the structure and function of DBRs is necessary.
DBRs in VCSELs are formed from alternating layers, each of which is one-quarter wavelength thick, of semiconducting materials having different refractive indices. The difference between the refractive indices of the low index layers and the high index layers must be of a certain amount before the DBR becomes sufficiently reflective to perpetuate, within the active area of the VCSEL, an optical resonance that results in an efficient lasing operation. In InP-based VCSELs, it is preferable to form the layers having the lower index of refraction from InP, while forming the layers having the high index of refraction from a semiconducting material having a compatible lattice structure that may be epitaxially grown on the InP layers. The use of such materials in the DBR allows the VCSEL to be grown epitaxially, for example by MBE (Molecular Beam Epitaxy), or MOCVD (Metal Organic Chemical Vapor Deposition). Examples of such materials that may be used as the high index layers include alloys formed from gallium (Ga), arsenic (As), aluminum (Al), antimony (Sb), indium (In) and phosphorus (P), such as AlGaInAs, GaInAsP, and AlGaAsSb. Other combinations of semiconducting materials that may be epitaxially grown over an InP substrate to form alternating high and low index layers for a DBR include AlGaInAs/AlInAs; GaInAsP/AlInAs and AlGaAsSb/AlGaAsSb. In this application, the term “InP-based VCSEL” encompasses all VCSELs grown on an InP substrate and including a DBR formed from any of the aforementioned alternating layers, or any other layers that may be epitaxially grown on an InP substrate.
Unfortunately, it is difficult to attain a large enough difference in the index of refraction between the high and low index layers when the aforementioned alloys are used. Since there is no way to lower the index of the materials forming the low index layers (such as InP or AlInAs), the only way to achieve a larger difference is to make the index of the high index materials higher by adjusting the mole fractions of the elements forming the high index alloy. This has the effect of decreasing the band gap of the high index materials so that it begins to approach the photon energy of the lasing wavelength of the VCSELs. However, the applicants have observed that if the band gap of the high index materials is decreased too close to the photon energy of the lasing wavelength, the resulting reflectivity can actually become worse due to absorption of the laser radiation by the high index materials. The applicants have further observed that if one attempts to avoid the laser radiation absorption problem by lessening the increase in the refractive index of the high index materials, the resulting VCSEL may not perform well over a broad range of temperature. The applicants believe that the fall off in performance is caused by a degradation in reflectivity of the DBR at higher temperatures since the optical absorption spectrum moves toward longer wavelength much faster than the lasing wavelength as the temperature is raised. On the other hand, if the index of the high index materials is not above a certain level, the reflectivity of the resulting DBR will be unacceptably low. Hence, the index of refraction of the high index materials may only be chosen within a narrow range in order to achieve a high-performance VCSEL over a broad range of ambient temperature. Because this narrow range for high performance varies with both the mid-wavelength of the VCSEL and the temperature of the device, up to now there has been no known way to easily and consistently manufacture high-efficiency, InP-based VCSELs.
Because of the difficulty in achieving the optimum amount of difference in the index of refraction in InP type materials, different semi-conducting materials have been used to form the DBRs in the prior art, such as alternating layers of GaAs/Al(Ga)As. Growth of thick layers of (Al)GaAs on InP results in a high density of defects and in morphological imperfections, due to the large difference between the crystal lattice constants of (Al)GaAs and InP. To avoid such defects propagating throughout the structure, a GaAs-based DBR has been mechanically transferred onto the InP-based active region. Unfortunately, such a wafer-bonding technique is difficult, requiring precise mechanical orientation between the GaAs-based DBR and the VCSEL wafers or films. Additionally, the bonding of a large area uniformly, without bubbles, is difficult. The reliability of the resulting VCSELs is easily compromised by an imperfect bonding process. Similar reliability problems are associated with the metamorphic growth of such GaAs-based DBRs onto InP substrates.
Clearly, there is a need for a method of fabricating InP-based VCSELs having DBRs of high reflectivity throughout a broad range of laser wavelengths, and in particular around 1.3 micrometer. Ideally, such a method would obviate the need for separate, wafer bonding steps, or for metamorphic-type growth.